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Counting cilia: Unusual checkpoint control of the atypical Cyclin O (Ccno) Counting cilia: Unusual checkpoint control of the atypical Cyclin O (Ccno) regulating number and formation of multiple motile cilia

Dr. Sebastian Arnold (Institute of Clinical and Experimental Pharmacology and Toxicology, University of Freiburg)

Cilia are microtubule based, hair-like organelles that extend from the apical surface of most cells. Most cells possess a solitary non-motile cilia, the so-called primary cilium, while specialized cells carry up to 300 motile cilia, thus referred to as multiciliated cells (MCCs). The coordinated back-and-forth motions of these motile cilia generate a directed fluid flow on epithelial surfaces, which have important physiological functions, such as the mucociliary clearance in the respiratory tract and the fluid transport in the ventricular system of the brain.

We recently identified Cyclin O (Ccno), which is specifically expressed in MCCs and there required for the generation of sufficient cilia numbers per MCCs to establish a functional fluid flow (Funk et al. 2015). Each ciliary axoneme extends from a so-called basal body, which is a specialized centriole. Thus, the differentiation of a MCC requires the formation of 200 – 300 centrioles, and one open question is how the cell manages to produce these precise numbers of centrioles. Moreover, Ccno-deficient MCCs produce reduced numbers of centrioles as the underlying cause for decreased cilia numbers on their apical surface. Interestingly, the transcriptional program that regulates centriole amplification in MCCs is downregulated once centriole formation is completed, and centrioles start to migrate towards the apical membrane for docking. However, in Ccno-deficient MCCs this transcriptional program fails to downregulated, but instead is further upregulated. This suggests that a feedback-loop on the transcriptional regulation exists, which stops the program once enough centrioles are formed. Using our Ccno mouse model we want to identify (1) by what mechanism MCCs ensure the formation of precise centrioles numbers, (2) if this mechanism includes a feedback-loop on the transcriptional program and (3) if Ccno might be directly involved in this counting mechanism.

To this end we are currently establishing a genetically approachable MCC culture system that can be used for gain- and loss-of-function, and reporter approaches to manipulate and microscopically follow the centriole formation process.